The present invention relates to an electrophotographic photoreceptor for electrophotography.
Amorphous silicon containing hydrogen H (to be referred to as a-Si:H hereinafter) has received a great deal of attention as a photoconductive material, and has been used in a variety of applications in solar cells, thin film transistors, image sensors, and electrophotographic photoreceptors.
The materials used as the photoconductive layers in conventional electrophotographic photoreceptors can be categorized as inorganic (e.g., CdS, ZnO, Se, or Se-Te) and organic (poly-N-vinylcarbazole (PVCZ) or trinitrofluorene). The a-Si:H has many advantages over the above-mentioned conventional organic and inorganic materials, in that it is non-toxic and does not require recovery, high spectral sensitivity in the range of visible light is guaranteed, and high surface hardness ensures high resistance to wear, and good anti-impact properties. For this reason, a-Si:H is receiving a great deal of attention as a promising electrophotographic photoreceptor.
The a-Si:H material has been developed as an electrophotographic photoreceptor on the basis of the Carlson system. In this case, good photoreceptor properties mean high dark resistance and high sensitivity to light. However, it is difficult to incorporate these two properties in a single layer photoreceptor. A barrier layer is arranged between the photoconductive layer and a conductive support, and a surface charge-retaining layer is formed on the photoconductive layer, to constitute a multilayer structure, thereby satisfying the two requirements described above.
The a-Si:H material for use as a photoreceptor is prepared by glow discharge decomposition, using a silane gas. During the fabrication process, hydrogen is incorporated in the a-Si:H film, whereby the electrical and optical characteristics thereof are changed greatly according to the change in content of hydrogen. As the amount of hydrogen incorporated in the a-Si:H film increases, the film's optical bandgap widens and its resistance increases. Along with the increase in resistance, the sensitivity to light having long wavelengths is degraded. Therefore, it is difficult to use such an a-Si:H film in a laser beam printer utilizing a semiconductor laser. When the content of hydrogen in the a-Si:H film is high, as described above, most of the components in the film can be those having bonding structures, such as (SiH.sub.2).sub.n and SiH.sub.2, depending on film formation conditions. In this case, the number of voids and hence, the number of silicon dangling bonds, increases, thereby degrading the photoconductive characteristics of the film. Under these circumstances, the film cannot be used as an electrophotographic photoreceptor. However, when the content of hydrogen in the a-Si:H film is low, the optical bandgap is narrow and the resistance lower. As a result, the sensitivity to light having short wavelengths is increased. A small content of hydrogen causes bonding of hydrogen atoms with the silicon dangling bonds, thus reducing the number of the silicon dangling bonds. For this reason, the mobility of photo-carriers is degraded, thereby shortening their lifetime. At the same time, the photoconductive property of the film is degraded and the film cannot be used as an electrophotographic photoreceptor.